The invention relates to an X-ray tube which includes a device for generating and focusing an electron beam on a target.
An X-ray tube of this kind is known, for example from DE 195 44 203. The electrons generated by an electron source (cathode) are accelerated in the direction of an anode in which they enter a conically constricted channel, the target being situated at the exit thereof. In this arrangement the electron beam is directed onto the target with a very small focus and a comparatively high electron density, so that X-rays are produced with a high efficiency.
This arrangement is in principle suitable for achieving a significant increase of the X-ray density (i.e. the number of photons emitted per unit of surface area of the target) in comparison with known X-ray tubes; however, such an increase is limited by the accompanying increase of the anode temperature. When this temperature reaches the range of the melting temperature of the anode material, the vapor pressure increases so that electric discharges could occur between the anode and the cathode.
Furthermore, the thermal conductivity of the anode decreases as the temperature increases. Consequently, the thermal conductivity of the electron focal spot in and through the anode material decreases and the temperature in the focal spot increases further, so that the melting temperature of the anode material is reached even faster and could be exceeded. This directly causes destruction of the anode surface. Therefore, it must be ensured that the focal spot temperature does not exceed a value of approximately 1500xc2x0 C. in X-ray tubes of this kind, so that the theoretically possible further increase of the X-ray density must be dispensed with to a significant extent.
Because a reduction of the anode temperature by radiant cooling due to the electromagnetic emission from the anode is practically non-existent, the only possibility is either to cool the anode, for example by means of a cooling medium (inter alia water), or to rotate the anode continuously so that the relevant region in the electron focal spot is heated only for a comparatively short period of time, after which it is allowed to cool down again.
This step enables the focal spot temperature to be increased to approximately 2200xc2x0 C. without the anode being damaged. Because the energy irradiated by thermal emission is proportional to the fourth power of the anode surface temperature, such rotary-anode tubes operate essentially with radiant cooling. The described steps, however, are either comparatively intricate or their effect is only limited.
Therefore, it is an object of the invention to provide an X-ray tube of the kind set forth whereby an essentially higher X-ray density can be achieved.
In an X-ray tube of this kind this object is achieved as described in claim 1 in that the target contains a material which is in the gaseous or vapor state at least in the operating condition of the X-ray tube and is contained under overpressure in a chamber which is at least partly permeable to electron radiation and X-rays.
If the target is then separated from the anode and substantially thermally insulated, the electron density in the focal spot of the electron beam can be significantly increased, so that a significantly higher X-ray density can be achieved without the anode temperature reaching inadmissibly high values.
The material contained in the chamber could be a noble gas having a sufficiently high atomic number, for example xenon which is gaseous in the operating condition as well as in the operating intervals. One embodiment describes the use of a heavy metal which may be solid or liquid in the operating intervals (i.e. at approximately room temperature) and is in a vapor state of aggregation in the operating condition (i.e. at comparatively high temperatures).
The entrance window offers the advantage that on the one hand the electrons passing through incur an energy loss of only approximately five percent, and that on the other hand the window is capable of withstanding pressure differences of up to 100 bar.
Another embodiment relates to coating the entrance window in conformity which offers the advantage that it will not be attacked and fogged by the high temperature plasma in the case of an unintentional increase of the operating pressure within the chamber.
The use of mercury in the quantity in another embodiment offers a particularly high efficiency.